1. Technical Field of the Invention
This invention relates to a terminal speed limiting device for an elevator and, more specifically, to improvements in the protection provided by the Normal Terminal Stopping Device in the hoistway.
2. Discussion of Related Art
It is known in the elevator art to define terminal zones at both ends of the elevator hoistway. The top landing of the building will normally be located within the top terminal zone as will the lower landing be located within the bottom terminal zone. It is desired that the elevator car stop normally at a top or bottom landing of the hoistway in such a terminal zone. As a safety measure, it is necessary to provide a number of backup means to ensure the elevator car does not collide with the mechanical hard-limits. Three levels of protection are usually provided when the elevator enters a terminal zone: the Normal Stopping Device, the Normal Terminal Stopping Device (or NTSD), and the Emergency Terminal Speed Limiting Device (or ETSLD). The present invention is concerned with NTSD which will take over from the Normal Stopping Device should the normal speed control signals fail to stop the car at the designated positions at the upper and lower ends of the hoistway. Two similar NTSDs are usually provided in the two terminal zones. One NTSD is installed at the bottom of the hoistway and one NTSD at the top of the hoistway. The NTSD system is designed to override the normal speed command signals and bring the car to stop at the terminal. It is also designed such that the NTSD terminal speed profile causes the slowdown pattern to be relatively smooth.
It is known in the art to mount a number of vanes in the hoistway and a sensor or sensors mounted on the car to read the vane identification for locating the position of car in the hoistway, and means to determine the velocity of the car in the terminal zone. For example, U.S. Pat. No. 5,637,841 (Dugan et al.) discloses an elevator system in which an NTSD system is used as a backup system. In particular, the NTSD system, according to Dugan et al, includes two operating modes: a monitor mode and a violation mode. The NTSD system normally operates in monitor mode where the NTSD speed profile has the same deceleration rate as the normal speed profile in the Normal Stopping Device. But when the velocity of the car exceeds the predetermined NTSD monitoring speed profile, or the maximum allowable NTSD speed profile for various car positions in the terminal zone during deceleration, the system substitutes the NTSD speed profile and switches to an NTSD violation speed profile for deriving subsequent NTSD speed values. The NTSD violation profile has a steeper deceleration slope than that of the profile in the monitor mode.
It is desirable to simplify the NTSD system so that only one operating mode will be used in the derivation of the NTSD speed profile. Furthermore, in the prior art NTSD designs, vanes are mounted in the hoistway using either a non-linear or linear spacing approach and this requires very tight control on vane spacing. In the limited space of the hoistway, the tight control of vane spacing sometimes becomes impractical. It is, therefore, desirable to provide an NTSD wherein the spacing criticality of vane installation can be relaxed.